A multi-system television complies with all major World broadcasting and video standards. Video standards include Phase Alternating Line (PAL), National Television System Committee (NTSC) and Sequential Color with Memory (SECAM). Most countries around the world utilize one of the three standards, but each standard is incompatible with each other.
The Vertical Frequency for PAL and SECAM is 50 Hz as specified in the standard (except PAL-M which is 60 Hz) while it is 60 Hz for NTSC standard. In a TV system, the Vertical Frequency is usually used for synchronizing other electronic circuit to avoid any interference to the display. This is often referred to as beating frequency effect. In addition, the Horizontal Frequency for PAL and SECAM is 15.625 KHz (except PAL-M which is 15.75 KHz) while it is 15.734 KHz for NTSC standard.
A lighting device for supplying power to a light source such as light emitting diodes or fluorescent lamps can be deriving from a DC/DC or DC/AC converters. These converters provide means of dimming control to adjust the amount of power delivered to the light source. One means of adjusting the power is to use pulse-width-modulation (PWM) technique (also called “burst mode dimming”) where the power delivered to the light source is modulated through a PWM control signal. The PWM frequency is normally between few tens of Hz to several thousand Hz.
To reduce display noise, there is a need to synchronize the Vertical Frequency signal to the lighting device. The electronic circuit in the light device has to detect and identify the Vertical Frequency in the TV system so that the PWM signal for adjusting the power to the light source is automatically adjusted to any of the standard used in any TV system.
Normally the operating frequency of a DC/DC or DC/AC power converters is in the range of tens of KHz to hundreds of KHz. It is typically not difficult to synchronize the operating frequency to the Horizontal Frequency in an integrated circuit in the lighting device. However, because the vertical frequency is relatively low, it is challenging to synchronize the PWM dimming frequency to the Vertical Frequency in the TV system.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art. Accordingly, it is intended that the claimed subject matter be viewed broadly, and be defined only as set forth in the accompanying claims.